Inverters or inverter units are devices used for producing variable frequency voltage from a direct voltage source. A typical application of an inverter is in a frequency converter, in which a rectifier unit rectifies AC voltage from a supplying network to a DC voltage to an intermediate voltage circuit. The intermediate voltage circuit or a DC bus consists of positive and negative terminals or rails and a capacitor bank connected between the terminals. The DC bus is further connected to the inverter unit for inverting the DC voltage of the DC bus to alternating output voltage. The alternating output voltage of the inverter is used typically for driving a load such as an electrical motor.
The main circuit of an inverter consists of series connected semi-conductor switches connected between the positive and negative intermediate voltages, and the point between the switches defines a phase output. By employing three of these series connections of switches in parallel, a three phase output is formed. Each output phase voltage can be selected to be either the voltage of the positive rail or the negative rail of the intermediate circuit.
There are many differing schemes for controlling the output switches for controlling the load in a desired manner. These control schemes or methods are typically based on feedback from some measured quantity, such as inverter current. The measurements of current are also used for purposes of protection, such as for overcurrent or short circuit protection.
The inverter currents are usually measured directly from the phase outputs for control purposes. This, however, requires at least two measurements in a three-phase system. The overcurrent protection can be carried out by measuring voltages of the output phases with respect to the negative rail of the intermediate circuit. If the voltage of a semiconductor switch, such as an IGBT, which is controlled conductive, is not small enough, it is assumed that the current of that IGBT is so high that the component does not stay in saturation. For this to occur, the current of the IGBT has to be much greater than twice the rated current. It is thus assumed that the output of the inverter is in short-circuit. The voltage measurement gives the voltages over the lower switches, i.e. switches connected to the negative rail. The voltages over the upper switches are obtained by subtracting the measurement results from the DC-link voltage.
Inverter currents can also be determined by measuring DC-current flowing in the intermediate circuit either in the positive or negative rail. Only one measurement is required. A problem relating to DC current measurement is that the current to be measured can be very high. Measurement is typically carried out by using a shunt resistor and by measuring the voltage drop in the resistor due to the current. Further, in high powers the intermediate circuit is designed to have low impedance. When a shunt resistor is added to the intermediate circuit rail, the rail has to be made from two pieces connected with the shunt resistor. The addition of the resistor increases the amount of impedance in an undesirable manner.